Add option to limit verbosity of build output

[pablochacin]

When running xk6 in a CI for generating multiple binaries (e.g. for multiple target GOOS and GOARCH) the output is noisy and makes hard to detect errors.

To improve this situation some current log outputs can be moved from INFO to DEBUG. Also a new environment variable XK6_LOG_LEVEL can be added to control amount of output generated.

[pablochacin]

Once #101 is completed, this functionality will be available as the k6foundry library supports verbosity levels

[szkiba]

Starting with xk6 v0.17.0, xk6 has two global flags that control the verbosity of logging:

Global Flags:
  -h, --help      Help about any command 
  -q, --quiet     Suppress output
  -v, --verbose   Verbose output

By default, INFO level logs are displayed. Using the -v flag, DEBUG level logs can be displayed. The -q flag suppresses INFO level logs and only WARNING or higher level logs will be displayed.

I am closing the issue as it is resolved.